1. Field of Invention
The present invention relates to water-resistant starch materials which are thermoplastic and can be processed into environmentally safe products, particularly moldings and thermoplastically produced sheets as well as cast sheets.
2. Description of the Prior Art
Starch esters, particularly acetates, have been known for a long period of time. A distinction is made between highly substituted starch acetates having a degree of substitution (DS) of 2-3 and low-substituted starch acetates having a DS up to a maximum of 1. Whereas highly substituted starch acetates have not yet gained any technical importance, low-substituted starch acetates are well-established commercial products.
Known acetates having a high degree of substitution (DS 2.5-3) are cellulose and amylose triacetates. According to literature the properties of films consisting of amylose triacetate are similar to those made of cellulose triacetate. The films were made of chloroform.
It is also generally known and described in numerous citations to produce starch acetates according to methods and processes common practice in the chemical industry, e.g. by using acetic anhydride, acetic anhydride pyridine, mixtures consisting of acetic anhydride and glacial acetic acid, ketene, vinyl acetate, and acetic acid, potato starch and corn starch being the predominantly used starches.
Relatively long reaction times and drastic reaction conditions had to be accepted to obtain highly substituted derivatives.
The strong decomposition of the starch molecules to give relatively short chains which do no longer have the film-forming properties typical of starch are especially disadvantageous in connection with these processes.
Little has been described about the production of high amylose starch acetate having a high degree of substitution. A regulation by Mark and Mehltretter is found in U.S. Pat. No. 3,795,670 and in the corresponding publication "Facile Preparation of Starch Triacetates" in the journal Starke, No. 3, pages 73-100, 1972. A purchasable high amylose corn starch from National Starch, which had an amylose content of about 70%, was used as the starch. The aspired complete acetylization was obtained by well-calculated selection of the catalyst and variation of its amount and a reaction time of about 5 hours by avoiding the above conventional processes and substance components used therein. The acetate obtained after 5 hours and having a degree of substitution of 3 could be cast together with dichloromethane solution into transparent flexible sheets. Although no other high amylose starches were used, it was assumed that, with equal course and results, the indicated process would generally be usable for high amylose starches and the obtained starch triacetates could furthermore be converted into fibers.
However, as expected the triacetates obtained by this process as well as sheets produced therefrom are not fully biodegradable or compostable within acceptable periods of time. Advantages over cellulose acetates have not been observed with respect to this point.